I have a similar problem on my 2014. The Parking Brake won't release without some coaxing. I found several complaints about this on MB World and the control part is available from several parts houses. So it appears that this is a common problem. Mine is still under the CPO Warranty so I took it in today for repair. When I get it back I'll post what they did to fix it. The P/N for 2013-2015 is Parking Brake Control - Mercedes-Benz (204-420-15-84) and the list price is 187. I found it for 158 from a dealer.

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Dave
88 300E 250K Sold
2000 C230K Totaled @104K
2003 ML320 156K Sold
2009 E350 4Matic Sport 46K Sold
2011 ML350 60K Sold
2014 GLK350 46k
2012 GL450 55K